Waste heat is often created as a byproduct of industrial processes where flowing streams of high-temperature liquids, gases, or fluids must be exhausted into the environment or removed in some way in an effort to maintain the operating temperatures of the industrial process equipment. Some industrial processes utilize heat exchanger devices to capture and recycle waste heat back into the process via other process streams. However, the capturing and recycling of waste heat is generally infeasible by industrial processes that utilize high temperatures or have insufficient mass flow or other unfavorable conditions.
Waste heat can be converted into useful energy by a variety of heat engine or turbine generator systems that employ thermodynamic methods, such as Rankine cycles. Rankine cycles and similar thermodynamic methods are typically steam-based processes that recover and utilize waste heat to generate steam for driving a turbine, turbo, or other expander. An organic Rankine cycle utilizes a lower boiling-point working fluid, instead of water, during a traditional Rankine cycle. Exemplary lower boiling-point working fluids include hydrocarbons, such as light hydrocarbons (e.g., propane or butane) and halogenated hydrocarbon, such as hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs) (e.g., R245fa). More recently, in view of issues such as thermal instability, toxicity, flammability, and production cost of the lower boiling-point working fluids, some thermodynamic cycles have been modified to circulate non-hydrocarbon working fluids, such as ammonia.
A synchronous power generator is a commonly employed turbine generator utilized for generating electrical energy in large scales (e.g., megawatt scale) throughout the world for both commercial and non-commercial use. The synchronous power generator generally supplies electricity to an electrical bus or grid (e.g., an alternating current bus) that usually has a varying load or demand over time. In order to be properly connected, the frequency of the synchronous power generator must be tuned and maintained to match the frequency of the electrical bus or grid. Severe damage may occur to the synchronous power generator as well as the electrical bus or grid should the frequency of the synchronous power generator become unsynchronized with the frequency of the electrical bus or grid.
Turbine generator systems also may suffer an overspeed condition during the generation of electricity—generally—due to high electrical demands during peak usage times. Turbine generator systems may be damaged due to an increasing rotational speed of the moving parts, such as a turbine, a generator, a shaft, and a gearbox. The overspeed condition often rapidly progresses out of control without immediate intervention to reduce the rotational speed of the turbine generator. The overspeed condition causes the temperatures and pressures of the working fluid to increase and the system to overheat. Once overheated, the turbine generator system may incur multiple problems that lead to catastrophic failures of the turbine generator system. The working fluid with an excess of absorbed heat may change to a different state of matter that is outside of the system design, such as a supercritical fluid becoming a subcritical state, gaseous state, or other state. The overheated working fluid may escape from the closed system causing further damage. Mechanical governor controls have been utilized to prevent or reduce overspeed conditions in analogous steam-powered generators. However, similar mechanical controls are unknown or not common for preventing or reducing overspeed conditions in turbine generator systems utilizing supercritical fluids.
Physical controllers and software controllers have been used to adjust independent aspects of turbine generator systems and process parameters. Such controllers may be utilized—in part—during a synchronous process or to avoid or minimize an overspeed condition. However, in the typical system, when a first controller is used to adjust a process parameter for manipulating a first variable, additional variables of the process generally become unfavorable and independent controllers are utilized to adjust different aspects of the process parameters while manipulating these variables. Such turbine generator systems that have multiple controllers are usually susceptible for failure and also suffer inefficiencies—which increase the cost to generate electricity.
What is needed, therefore, is a turbine generator system, a method for generating electrical energy, and an algorithm for such system and method, whereby the turbine generator system contains a control system with multiple controllers for maximizing the efficiency of the heat engine system while generating electrical energy.